The present invention relates to a quartz glass jig excellent in the plasma etching resistant characteristics, which is used for an apparatus generating plasma.
Recently, for the surface treatment of a semiconductor element such as a silicon wafer, etc., an etching treatment utilizing plasma has become frequently used. As the plasma etching method, there are a method of introducing a halogen-base corrosive gas such as a fluorine-base, a chlorine-base, etc., into a plasma-generating chamber. On the other hand, introducing a microwave via a microwave-introducing window to form the plasma of the above described halogen-base corrosive gas existing in the chamber, and treating a semiconductor element, a method for introducing a halogen-base corrosive gas and a microwave in a hanging bell-form quartz glass-made bell jar to form the plasma of the halogen-corrosive gas, and treating a semiconductor element (for example depositing an insulating thin film on the surface of the semiconductor element) is known. In each treatment, an apparatus of generating plasma is used. Also, for the apparatus, quartz glass jigs are mostly used as a window material for introducing a microwave, a bell jar is used as a plasma-generating chamber and a ring for shielding the apparatus. As a material for these jigs natural quartz glass has been mainly used owing to the good electric characteristics and the high purity. The surface of the jig, which is brought into contact with plasma, is roughened by mechanical working to stabilize the etching speed and prevent deposits from being released. For above-described roughening a grinding work using a grindstone with ceramic abrasive grains, diamond abrasive grains, silicon dioxide abrasive grains, etc., are used, also a sand blast work using a powder such as a green carbon powder, a silicon dioxide powder, a ceramic powder, etc., has been employed, but by the mechanical working method, simultaneously with roughening the quartz glass surface, the generation of a microcrack layer occurs. The layer or microcrack portion is attacked by radicals formed in the plasma, consequently the microcrack portions only are etched to form abnormally large holes, and in the worst case, there is a fault that the breakage of the quartz glass jig starts from that portions. Also, recently, it has been confirmed that in the above-described plasma, together with the generation of the ions and radicals, abnormally intense ultraviolet rays and electron rays are emitted from the gas, and it has been found that these rays deteriorate the surface of the quartz glass jig, the deteriorated portions becomes the sources of generating particles, and the generated particles deposit on the surface of a semiconductor element to give secondary bad influences.
Furthermore, there is a problem that impurities sometimes enter the microcracks formed by the mechanical working method, and the impurities are volatized at the treatment of a semiconductor element to contaminate the semiconductor element.
Also, natural quartz glass sometimes contains many bubbles and thus has the fault that even when the surface is roughened by a chemical method, which does not generate microcracks, the surface thereof is deteriorated by usually intense radiations and electron rays emitted in the plasma, when the bubbles are released and exposed on the surface, the deterioration greatly proceeds at the bubble portions to generate particles, which attach onto the surface of a semiconductor element to deteriorate particularly the dielectric breakdown characteristics.
The object of the invention is to provide a quartz glass jig excellent in the plasma etching resistant characteristics, which gives less abnormal etching and less deterioration of the surface of the quartz glass, and does not give contamination of semiconductor elements by the entrance of impurities.
As the results of making intensive investigations about a quartz glass jig used for an apparatus of generating plasma, the present inventors have found that by making the surface roughness Ra of the quartz glass surface of the jig in the range of front 5 xcexcm to 0.05 xcexcm, the number of microcracks of the surface 500 microcracks/cm2 or less, and increasing the content of a hydrogen molecules in the quartz glass to at least 5xc3x971016 molecules/cm3, a quartz glass jig, which shows less deterioration of the jig surface by etching by plasma and the abnormally intensive ultraviolet rays and electron rays, and does not give bad influences onto a silicon wafer by the generation of particles and impurities.
Furthermore, it has also been found that by establishing the content of bubbles in the above-described quartz glass to 0 or 1 as the bubble classification by DIN, the double refraction amount of the quartz glass to 70 nm/cm or lower, and fictive temperature of the quartz glass to the range of from 800 to 1200xc2x0 C., a quartz glass jig, which gives a greatly less deterioration by etching by plasma and absolutely intensive ultraviolet rays and electron rays, gives less entrance of impurities by releasing of bubbles, and can carry out well the treatment of semiconductor elements, is obtained, and the present invention has been accomplished.
The present invention attaining the above-described object is a plasma resistant quartz glass jig that is used for an apparatus generating plasma, characterized in that the surface roughness Ra of the quartz glass surface is from 5 xcexcm to 0.05 xcexcm, the number of microcracks of the surface is 500 microcracks/cm2 or less, and the hydrogen molecule concentration in the quartz glass is at least 5xc3x971016 molecules/cm3.
The quartz glass jig of the invention is the jig prepared by a natural or synthetic quartz glass, and the inner surface thereof, which is brought into contact with plasma, is roughened in the range of from 5 xcexcm to 0.05 xcexcm as the surface roughness of Ra for stabilizing the etching speed and preventing release of deposits. When the surface roughness Ra exceeds 5 xcexcm, the radicals and ions generated in plasma locally attack the concaved portions and thus abnormal etching proceeds. Also, when the surface roughness Ra is less than 0.05 xcexcm, there is a danger that secondary products (for example, particles, etc.) formed by a plasma are liable to be released from the surface and they deposit on the surface of the semiconductor element (e.g. silicon wafer) to largely reduce the characteristics of the semiconductor element.
Also, the quartz glass jig of the invention is the jig wherein the number of microcracks of the surface is not more than 500 microcracks/cm2. When the number of the microcracks of the surface of the jig exceeds the above-described range, the proceed of etching becomes faster by attacking by the radicals generated in the plasma, the microcracks become large, and in the worst case, the breakage of the quartz glass starts from that portion. The quartz glass jig wherein the above-described number of the microcracks is not more than 500 microcracks/cm2 can be prepared by a chemical treatment such as the method of immersing a quartz glass jig in a solution of hydrofluoric acid, ammonium fluoride and acetic acid to deposit fine particles of ammonium silicofluoride as described in Japanese Patent Laid-Open No. 273339/1998, the method of forming the thin film of silicone, etc., and applying an etching treatment with an aqueous solution of hydrofluoric acid as described in Japanese Patent Laid-Open No. 106225/1999, etc. In the case of roughening by the chemical treatment, it is important to control the reaction of the etching chemical liquid and the quartz glass, and particularly, in the case of a synthetic quartz glass, the temperature management of the chemical liquid is necessary and it is better that the temperature of the chemical liquid is controlled in the range of 20xc2x0 C. xc2x12xc2x0 C. When the temperature is outside of the above-described temperature range, the surface roughness Ra of the quartz glass surface cannot be made from 5 xcexcm to 0.05 xcexcm. The number of the microcracks of the quartz glass surface can be measured by observing the microphotograph. Of course it is even better when no microcrack layer exists on the surface of the quartz glass jig of the invention.
Furthermore, in the quartz glass jig of the invention, the hydrogen molecule concentration is at least 5xc3x971016 molecules/cm3. By having the hydrogen molecule concentration, the formation of the Exe2x80x2 center absorption bond is restrained, the inner strain in the network structure of the quartz glass is removed, the break of the Sixe2x80x94O bond by the abnormally intense ultraviolet rays and electron rays in the plasma becomes less, the increase of the density of the quartz glass is restrained, and the generation of large microcracks and the generation of particles become less. The above-described hydrogen molecule concentration is obtained by controlling the hydrogen molecular weight in a flame at the production of the quartz glass or by treating the quartz glass in a hydrogen gas atmosphere of a normal pressure or under pressure at a temperature of from 100 to 900xc2x0 C. for from 1 to 100 hours, and the measurement is by a laser Raman method.
In addition to the above-described matters, in the quartz glass of the invention, it is better that the content of bubbles is 0 or 1 as the bubble classification by DIN. When the quartz glass contains bubbles exceeding the above-described range, the bubbles are eroded by the radicals and the ions generated in the plasma and released to become precursors of abnormal holes, and the deterioration further proceeds by the abnormally intense radiations and electron rays in succession thereof to form large holes and generate particles. The term that the content of bubbles is 0 or 1 as the bubble classification by DIN described above is that according to DIN (Deutsche Industrie Norm) 58927 which is herein incorporated by reference, the total cross sections (cm2) of the bubbles existing in 100 cm3 are from 0 to 0.03 when the classification is 0, and are from 0.03 to 0.10 when the classification is 1. As the quartz glass wherein the bubble classification by DIN is 0 or 1, there are commercially available HERA LUX and SUPRASIL (manufactured by Shin-Etsu Sekiei K. K. and Heraeus Quarzglas GmbH and Co. KG).
In addition it is also important that the quartz glass is homogeneous such that the double refraction amount is not higher than 70 nm/cm. Also, when the double refraction amount of the quartz glass exceeds the above-described range, the increase of the density of the quartz glass proceeds by the abnormally intense ultraviolet rays and electron rays irradiated to the twisted portion and microcracks become large. For effectively preventing the increase of the density of the quartz glass by the abnormally intense ultraviolet rays and electron rays, it is better that the fictive temperature of the quartz glass is established to the range of from 800 to 1200xc2x0 C. By establishing the fictive temperature to the above-described range, the density of the quartz glass is lowered to relax the structure, by the irradiation of the abnormally intense ultraviolet rays and electron rays, defects are hard to generate and the increase of the density can be prevented.
In the quartz glass jig of the invention, a fine roughness is formed on the surface as described above, which largely acts on the transmittance of ultraviolet rays. When the transmittance is small, the quartz glass surface is deteriorated by ultraviolet rays. Particularly, The Sixe2x80x94O bond of the quartz glass is broken by abnormally intense ultraviolet rays and electron rays simultaneously generating with the radicals and the ions in the plasma to form a defect, the density becomes high, microcracks become large, the etching speed is greatly accelerated, and large holes are formed. For preventing the occurrence of the deterioration of the quartz glass by ultraviolet rays, it is better that the inner transmittance of the quartz glass is at least 80% as the transmittance of ultraviolet rays, and the apparent transmittance is from 30 to 85%. It is impossible that the appearance transmittance exceeds 85% because the surface becomes rough, and also, when the apparent transmittance is less than 30%, it cannot be prevented surface of the quartz glass from becoming precise. The above-described inner transmittance means the transmittance, which does not contain the reflection loss at the outer surface of the quartz glass, and the transmittance containing the reflection loss is called an apparent transmittance.
As the quartz glass, wherein the bubble classification by DIN is 0 or 1 and the double refraction amount is in the above-described range, there is suitably a synthetic quartz glass obtained by hydrolyzing a silicon compound in the vapor phase. Also, the establishment of the fictive temperature of the quartz glass is carried out by heating the quartz glass to a temperature of from 800 to 1200xc2x0 C. for a definite time. The measurement of the fictive temperature is by a laser Raman method.
The quartz glass jig of the invention is prepared by a natural or synthetic quartz glass as described above, and a synthetic quartz glass, which has less content of bubbles and can be easily obtained as a quartz glass having a high homogeneity, is particularly preferred.